The present invention is directed to a system and method for obtaining precise survey-grade position data, particularly in areas or zones where precise GPS data cannot be obtained, due to natural or man-made objects such as trees and buildings.
An open technical milestone within the geomatics community is the extension of precise positioning with a GPS survey instrument into areas where some or all of the signals from the GPS satellites are perturbed. This problem has several dimensions that include the technical dimension, the economic dimension and the man-machine interface (MMI) dimension.
The economic dimension is the GPS survey instrument's value proposition that justifies its price of $35K-50K. The value proposition is the significant improvement in efficiency over alternative precise positioning methods that include conventional total stations (CTS), automatic total stations (ATS) and fan lasers. A GPS survey instrument requires one operator and can operate over a broad area of up to 10 km away from its base receiver without relocation of infrastructure. A CTS selling for around $10K requires a crew of two operators and has a range of a few hundred meters. An ATS selling for around $45K requires only one operator, but has a range limit similar to a CTS. Both require a fairly elaborate setup per location. Fan lasers have even shorter range limits and require elaborate and time-consuming installations. Consequently the GPS survey instrument can provide an excellent value proposition so long as it delivers reliable centimeter-level positioning needed for most survey-grade applications.
A surveyor using only GPS is normally able to provide a specified surveyed position accuracy as that from competing survey instructs (examples are CTS, ATS, fan lasers, traditional rod-and-chain). This accuracy is on the order of one centimeter (cm) for precision land survey. It ranges from 10 cm to one meter for lower accuracy survey applications such as cadastral survey 5, geographic information system (GIS) and seismic survey.
The GPS survey instrument's accuracy reliability diminishes when one or more signal lines of sight pass through foliage. Hence the current generation GPS survey instrument is unusable near trees or buildings that can shade, reflect or refract the GPS signals. Such an area is hereafter called a precise-GPS-denied zone. More precisely, a precise-GPS-denied zone is a zone or region where a GPS surveyor cannot provide the required accuracy in locating a point in three dimensional space (for example, accuracy may degrade from 1 cm to 3 cm in a precision land survey due to signal refraction from nearby foliage or buildings). This describes the fact that although a GPS receiver may continue to provide a position solution, it cannot reliably provide a precise survey-grade position solution which has centimeter-level accuracy. If an operator is forced to use a CTS or ATS as frequent backup because of extensive foliage in a job area, then he will likely use the CTS or ATS for the whole job, and not use the GPS survey instrument. The value proposition of the GPS survey instrument thus diminishes in the presence of foliage and/or signal obstructions. Consequently some method of preserving the value proposition is needed, which in turn requires a technical solution to accuracy preservation in precise-GPS-denied zones.
This leads into the technical dimension of the problem. Two possible approaches are to (1) use the compromised GPS signals to maintain centimeter accuracy via a TBD signal processing algorithm, and (2) navigate through precise-GPS-denied zones coverage using some other positioning means. The solution should provide survey-grade precision data to locate objects or targets within the zone.